In general, a turbomachine comprises an upstream low pressure (LP) compressor and a downstream high pressure (HP) compressor. Both compressors are axial and they rotate at different speeds, often requiring air to be taken from their interface. When idling, in order to protect the upstream compressor from pumping as generated by the mismatch between the speeds of the two compressors, discharge valves are opened to eject the excess air flow generated upstream that cannot be absorbed downstream.
Activation and deactivation of the discharge valves is under the control of a full authority digital electronic control (FADEC) system of the turbomachine.
FIG. 5 shows an example of a prior art control system 115 for controlling the discharge valves 109 of a turbomachine.
That system 115 comprises an electronic control unit (ECU) 117 and a pneumatic control unit (SBU) 118 connected via control lines 145 to N discharge valves 109. The pneumatic unit 118 is fitted with N solenoid valves 129a, 129b, . . . , 129n, having one solenoid valve per discharge valve 109.
A discharge valve 109 that is deactivated (i.e. open) corresponds to the valve being in a rest position.
In order to activate a discharge valve 109 (i.e. close it), the electronic control unit 117 powers the solenoid valve 129a, 129b, 129c, . . . associated therewith which connects the high pressure air 137 at the outlet of the HP compressor to the control line 145 of the discharge valve 109.
The discharge valve 109 has a chamber 143 containing a piston 147 acting against a spring 146. Thus, when the chamber 147 of the piston is fed with high pressure air, the piston 147 moves, compressing the spring 146, and the rod 148 of the piston moves a drive linkage 149 that serves to cause an air exhaust channel 10 to be closed by an internal member 151 of the discharge valve 109.
To reopen the discharge valve 109, the electronic control unit 117 ceases powering the solenoid valve 129a, 129b, 129c, . . . , thereby causing the chamber 143 of the piston to be connected to ambient pressure 139. The spring 146 then returns the piston 147 to the rest position so as to re-open the internal member 151 via the drive linkage 149.
That system nevertheless presents numerous drawbacks. The system is quite complex and the control of the discharge valves 109 frequently busies the electronic control unit 117 which is also used for controlling other important functions of the turbomachine.
Another drawback is the fact that the system does not have any means for directly detecting the position of the discharge valve 109 in order to identify a breakdown (a valve blocked in an open or a closed position). In present systems, a failure is detected by observing the behavior of the turbomachine, which reduces the time available for taking corrective action to prevent pumping. Furthermore, the maintenance assistance functions cannot locate the faulty discharge valve 109.